The present invention relates to a device for detecting and quantifying an unbalance between electric paths, particularly for a touch detection system.
The present invention thus relates in particular to the field of devices known under the common term of touchpad, i.e., devices for the detection of a touch or the presence of a body, notably a tactile approach to a surface that may consist of a pad, i.e., a tablet or support for tactile imprint that under its surface has a series of conducting contact pieces arranged into various regular patterns (line, array, diamond-shaped pattern, honeycomb, spiral turns, maze, etc.) or may consist of a screen, which belongs to the allied field of touchscreens, that is, screens including devices that are sensitive to the approach of a finger or of an object such as a pen or more generally a condensed body.
There are known devices for the acquisition or recognition of tactile imprint that are based on electric and notably capacitive measuring principles, as well as detectors functioning by capacitive detection or switches functioning by proximity detection that also use capacitive detection, and hence are considered to be based on electrostatic principles.
In general, the detection principle of such devices is based on the implantation of large contact pieces or tracks on the two sides of a printed circuit, which sides form face-to-face conducting surfaces and therefore, from an electrostatic point of view, the two adjacent plates of a plane capacitor separated by a dielectric constituting the support of the printed circuit. When a finger approaches this surface, it perturbs the electric field and may for this reason be detected by adequate electronic circuits existing in many forms and variants.
One of the essential problems of this kind of devices is their sensitivity to electromagnetic perturbations, all the more as the absolute values of capacitance of these capacitors are generally very small (typically of the order of picofarad, such as 1 pF), and thus highly delicate to measure electronically, since they are readily perturbed by parasitic capacitances that are part of the measuring circuits, and by electromagnetic perturbations. This holds all the more as the proximity of a finger to the capacitor contact pieces of a touchpad will not substantially modify its value of capacitance. As a numerical illustration, the touch by a finger or presence of a body behind the capacitor plates of a plane capacitor with a capacitance of the order of 1 picofarad will modify its value by only 10%, which brings its capacitance to 0.9 pF or 1.1 pF. It is particularly delicate to measure electronically a capacitance variation of 0.1 pF (or 100 fF), and the tactile detection devices are known to be particularly capricious and uncertain with respect to the detection of a touch or tactile contact.
These devices have the disadvantage that the tactile approach behind the capacitor plates of a plane capacitor constituted by two conducting contact pieces on the two faces of a support will in fact induce a very slight variation in the value of capacitance, namely a variation that is slight with respect to the total value of capacitance, since the capacitance is little affected by modifications of the medium outside the interval separating the two capacitor plates, and that is slight in the absolute, since the values of capacitance between two thin conducting tracks with dimensions distinctly inferior to those of a finger will themselves amount to small absolute values.
A solution available to make a touch detection device less sensitive to the electromagnetic environment consists in detecting in a differential way the balance between first and second electric paths that are linked to the conductors or electrodes of the touch surface. Circuits capable of making such a differential detection are described in the documents WO 96/18179 and US 2002/0039092. They comprise an amplifier which receives the electrical paths at its inputs and which outputs a signal proportional to the difference of potential between these paths. The output signal must be treated analogically and then digitized in order to allow the electric unbalance to be quantified. These circuits are relatively difficult to realize, are sensitive to the environment (stability of the supply voltage, temperature, etc.), and generate noise.